1. Field of the Invention
This patent specification relates to testing in sequestration wells. More particularly, this patent specification relates to systems and methods for vertical isolation testing of the annular region between the casing and formation in sequestration wells.
2. Background of the Invention
Sequestration otherwise known as geo-sequestration or geological storage, involves injecting a material, such as carbon dioxide, directly into underground geological formations. Declining oil fields, saline aquifers, and unminable coal seams are included in potential storage sites. CO2 has been injected into declining oil fields for more than 30 years to increase oil recovery. This option is attractive because the storage costs are offset by the sale of additional oil that is recovered. Further benefits are the existing infrastructure and the geophysical and geological information about the oil field that is available from the oil exploration. Oil fields have a geological barrier preventing upward migration of oil. It is believed that many such geological barriers are also sufficient as a long-term barrier to contain the injected CO2.
A significant challenge with CO2 sequestration is the prevention of leakage. To prevent leakage at the injection wells or other wells where potential leakage can occur such as current or disused production wells and/or monitoring wells, isolating cement is provided in the annular region between the well casing and the subterranean formations. Monitoring the isolating cement in the annular region helps to ensure that quality isolation is maintained over time. Techniques are known for evaluating isolating cement in conventional oil wells, such as cement bond logging, which measures the bond between the casing and cement, makes measurements using acoustic, sonic, and ultrasonic tools. However, there is a fundamental time-scale difference between the productive lifespan of a conventional oil well which can be a number or perhaps dozens of years, and the lifespan of a sequestration well which can be longer, possibly hundreds of years. Due to this difference in time scales, the cement evaluation techniques known for conventional oil wells may not be sufficient for monitoring cement quality over time for sequestration wells. Additionally, when CO2 is placed in subterranean formations it tends to dissolve or disassociate to form carbonic acid. The carbonic acid is potentially detrimental to the isolating cement in the annular region. Therefore, additional methods and tools for evaluating the isolation quality of the annular region in cased sequestration wells are needed.